The development of low emission, high performance combustors is an area of much current interest. In particular, NOx emissions from numerous major combustion sources such as gas turbines for power generation and aircraft propulsion as well as a variety of boilers, furnaces and heaters are a major environmental problem. Proposed future reduction of allowable NOx emission levels will only increase the need for effective control strategies. Consequently, the reduction of these emissions in an efficient and cost effective manner will have a major economic impact.
It is well known in the art that NOx emissions from combustors are largely determined by combustion temperature, i.e., lower combustion temperatures result in a exponential decrease in NOx emission levels. Coupled with this is the fact that the extent of reduction of gas temperatures in the combustor can be limited by the onset of combustion instabilities. These instabilities generally lead to incomplete combustion of the fuel, unstable flames, the release of higher quantities of carbon monoxide (CO) and unburned hydrocarbons (UHC), and in the limit, flame extinction. High amplitude pressure oscillations in the combustion chamber, driven by combustion heat release, can also be present. Under the right conditions, the amplitude of these pressure fluctuations increases and can, at a minimum, result in a degradation of combustor performance. In the limit, the amplitude of the pressure fluctuations can be sufficient to cause significant damage to combustor hardware and burner components. Whether the combustor operates in a stable mode or an unstable mode is determined by numerous factors. These can include, but are not limited to, fuel type, fuel/air ratio, inlet pressure, combustor geometry, combustor throughput, and the coupling between combustion chamber design and flame heat release.
Combustion consists of a chemical reaction between a mixture of fuel and air to release heat. The term equivalence ratio is often used to identify the actual quantities of fuel and air provided. As used herein, the term is defined as the ratio of fuel to air provided divided by the stoichiometric ratio of fuel to air. The stoichiometric ratio is achieved when the proper amount of air is provided to completely consume all the fuel. Thus, an equivalence ratio of unity corresponds to an amount of air exactly equal to that needed to consume all the fuel while an equivalence ratio less than unity indicates excess air, i.e., a fuel lean condition. Typically maximum combustion temperatures occur at near stoichiometric conditions (near an equivalence ratio of unity). As the equivalence ratio exceeds or becomes less than unity the combustion temperature decreases with a concomitant decrease in NOx emissions since these emissions are a strong function of temperature, increasing exponentially with increasing temperature.
Most current combustors operate in a non-premixed mode where the fuel and air are introduced separately. An advantage of this mode is that potential safety problems such as flame flashback, which can occur when the fuel and air are premixed prior to combustion, are eliminated. In the non-premixed mode combustion occurs predominately at stoichiometric conditions where the maximum temperatures are produced. As discussed above, this high temperature combustion maximizes the production of NOx. The addition of diluents, such as nitrogen or water vapor, can be an effective control strategy for NOx emissions since they tend to lower the combustion temperature. However, this emissions control strategy is limited by the finite operating range of a combustor. At high dilution levels the flame temperature becomes sufficiently low that the heat loss rate exceeds the combustion heat release and the flame can no longer sustain itself. This condition is referred to as the “flame blowout limit” leads to flame extinction and provides an upper boundary for the amount of diluent addition. It is known in the art that because of the wide flammability limits and faster burning rates of hydrogen, the addition of hydrogen to a conventional hydrocarbon fuel, such as methane or natural gas, significantly improves the low temperature combustion characteristics and extends the lean fuel blowout limit so that lower temperature operation can be achieved. A more detailed discussion of the effects of hydrogen on combustion characteristics can be found in co-pending application Ser. No. 10/091,044, filed Mar. 4, 2002, entitled “Method for Controlling Lean Combustion Stability”.
Several approaches are currently used in gas turbine combustion systems to reduce NOx emissions. These are typically passive control approaches that include changes in combustion chamber design, variable geometry designs, lean-premixed combustion, staged combustion designs selective catalytic reduction (SCR) with ammonia addition and modification of the injected fuel distribution pattern by modification of the fuel injector design or the air inlet pattern, among others. These approaches are often costly and limited in terms of their effectiveness.